Mechanical or structural vibration in mechanical systems typically can result in auditory noise. The structural vibration, such as from a large surface set in vibratory motion, substantially simultaneously sets into motion the acoustic medium, e.g. air or water, around the vibrating member. Thus, when a structure in a mechanical system is set into motion by a mechanical source of vibration, it in turn causes a propagation of an acoustical signal, i.e. noise, in the surrounding air. Furthermore, vibration generated in one place in a mechanical system is often transmitted elsewhere through transmitting structures in the mechanical system, and is undesirably further converted into audible or airborne noise by radiating structures. Accordingly, noise levels in mechanical systems, such as large helicopters, can be very high in part due to vibrations originating as structural noise, e.g. in meshing gears in the transmission. The low mass of mechanical systems, such as helicopter transmissions, accentuates the problem of structural vibration and noise. Resultant noise can have problematic implications in other systems in the aircraft, including communication systems. Typical vibration frequencies of helicopter transmissions and other aircraft equipment in the 1 to 2 kHz range can cause significant interference with the audibility of speech, which has many frequency components in that range.
Past efforts to reduce structural vibration and/or noise in helicopter transmissions have focused on passive techniques such as: transmission tuning to shift the frequency of vibrations to an innocuous range; isolation to limit and/or preclude resonances caused by the interplay of vibration from different sources; and absorption to muffle or absorb noise by way of noise barriers. However, these approaches do not produce sufficient reductions in vibration and noise.
Active feedback methods are being investigated and applied as a promising alternative approach to reducing narrow-band vibration and noise in environments where passive techniques have proven inadequate (e.g. at higher frequencies, for example, greater than 400 Hz). Typical active feedback methods require transducers or sensors to sense noise levels at frequencies of interest and actuators to deliver cancellation signals at corresponding frequencies, typically 180 degrees out of phase with the noise sensed. However, a lack of availability of high force-to-mass ratio actuators significantly limits the applicability of such active vibration cancellation schemes in weight sensitive applications, such as helicopters or other aircraft. That is, the availability of light weight actuators that can deliver the high forces necessary for high frequency active noise cancellation is very limited. In particular, of the components required in active noise cancellation systems (e.g. sensors, actuators, controllers and electronics), it is the mass of the actuator(s) that is the critical consideration, for example in helicopter applications.
Reaction Mass Actuators (RMAs) are known for delivering forces for active noise cancellation. RMAs are typically designed and configured to be mounted on vibrating structures to deliver cancellation signals on the structures, and to substantially preclude the transmission of vibration through the structures. Electromagnetic voice coil RMAs are known which typically use springs and masses to produce a resonant system that achieves relatively high force densities. However, the resonant system in electromagnetic voice coil RMAs produces high force in a narrow band of frequencies, and almost no force at non-resonant frequencies. Accordingly, electromagnetic RMAs have a very narrow band of useful frequencies, i.e. they are of limited applicability in applications where a broader range of frequencies is involved. Further, the resonant systems in some known electromagnetic voice coil RMAs include solid components that engage each other causing "chatter" and/or harmonic noise at frequencies other than that of the output force. Extraneous noise generated by known voice coil RMAs works counter to the purpose for which the devices are generally employed. Additionally, disadvantageously, many known electromagnetic voice coil RMAs are open designs with actuators that are not readily or easily sealed from the elements.
Some known RMAs incorporate piezoelectric or electrostrictive materials to effect actuation. However, such known RMAs have disadvantages in certain applications. Specifically, high voltages on the order of 1,000 volts are typically required for piezoelectric or electrostrictive actuators, presenting numerous issues associated with power sourcing. The power sourcing requirements mitigate against the use of such actuators in weight sensitive applications.
Magnetostrictive materials have properties similar to piezoelectric and electrostrictive materials and have been used in active actuator applications. The magnetostrictive materials used change shape (strain) in the presence of magnetic fields by up to 2000 .mu.-strain (ppm) at room temperatures, which is twice the strain available in the best piezoelectric or electrostrictive materials. With magnetostrictive materials, achieving such large strains does not require the use of high voltages.
Another advantage of magnetostrictive materials in actuator applications, over competing technologies such as piezoelectric, is a lack of fatigue phenomena. There is no known fatigue mechanism in known magnetostrictive materials and therefore no time or cycle dependant lifetime. This is in contrast with piezoelectric materials, which suffer from microfractures that develop during use and lead to breakdown under the high voltages required for implementation of piezoelectric actuators.
Terfenol-D.RTM. magnetostrictive material, produced by ETREMA Products Inc., a Subsidiary of EDGE Technologies Incorporated, is a room temperature, high performance magnetostrictive material which increases in length when a magnetizing field is applied parallel to the material's drive axis. Magnetostrictive strain depends only on the magnitude of the magnetizing field, not its sign. Bi-directional actuation can be achieved by inclusion of a bias field such as produced by a coil or permanent magnets, to bias the Terfenol-D material to elongation of some degree, e.g. one half, of its linear range enabling actuation about a bias point. Some sort of mechanical prestress is generally desirable, as it has been determined that the best performance of Terfenol-D is achieved by configuring the magnetostrictive material with mechanical prestress or compressive preload.
Terfenol, however, is relatively weak in tension and is known to be brittle. These properties can affect actuator reliability in improperly designed devices incorporating such material. For practical application, magnetostrictive actuators typically require sophisticated mechanical designs to ensure longevity of the actuator. Elaborate mechanical features designed into known magnetostrictive actuators to isolate the Terfenol rod from shear and tensile stresses, and to overcome weaknesses due to the physical properties of Terfenol, add significant cost and weight to the actuators. Generally, known magnetostrictive actuators have limitations and present significant design issues when subjected to significant loads.
Furthermore, known Terfenol actuators have limited applicability in RMA applications. The mechanical design of known Terfenol actuators is such that they tend to be high friction devices with a significant amount of static friction (stiction), and damping in the actuator. In addition, they have a low moving mass, high actuator mass, and low power density that makes them unusable in most reaction mass applications.
Terfenol magnetostrictive material has been used in actuators other than RMAs, such as actuators described in U.S. Pat. Nos. 5,231,887 and 5,406,153. The mechanical configurations described therein, like other known magnetostrictive actuators, are not suitable for RMA applications in that they are high friction devices with a significant amount of static friction (stiction), and damping in the actuator. The designs, which are clearly not intended for RMA applications, include several solid components that will interact with each other to generate harmonic noise at frequencies other than that of the output force. Furthermore, in these illustrative implementations of magnetostrictive actuators, there is no concern for maximizing the mass of the actuator for the purpose of power density maximization.